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additional stress upon the solder mask and could potentially lead to adhesion loss, color change, embrittlement, or cracking. 33.4.3.3 Low Halogen. The low-halogen requirement came about due to concerns about incineration of scrap PCBs generating dioxins under some conditions, and originally centered on some brominated flame retardants used in base laminates. It progressed from that into an additional concern over the chlorine content. This requirement is sometimes referred to as halogen-free, but in reality, the true requirements place a limit on the amount of bromine, chlorine, and their sum in the content of the material. Current low-halogen requirements on the cured solder mask are as follows:
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Although solder masks do not typically contain flame retardants, and hence no bromine, they often contain chlorine. The chlorine generally comes from pigments that give the mask its desired color, and also from residual catalyst from the resin (solder mask raw material) manufacturing process. A number of manufacturers offer low-halogen solder mask materials that meet the specified criteria.
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Circuit Density Issues As the pitch of circuitry and packages has decreased, it has also generated the demand for smaller holes and finer solder mask features. 33.4.4.1 Hole Size. As mentioned in Sec. 33.2.1.2, small hole sizes are a significant challenge for the solder mask process. All application processes introduce some solder mask into the holes, and this material must be removed (developed) from the holes before curing. There are two primary considerations in achieving finished PCBs with no mask in the holes. First, it is easier to achieve mask-free holes when less solder mask is introduced into the holes during the application process. In general, spray applications put the least mask into holes, and single-sided screening introduces the most. Curtain coating and simultaneous double-sided screen printing produce more moderate levels of solder mask in holes. The ability to remove all solder mask from the holes is dependent upon the solder mask material itself, the tack drying, and the developing processes. Some solder masks inherently develop faster than others, so the choice of products has a significant impact on development. If the solder mask is subjected to an overly aggressive (too hot and/or too long) tack drying process, the mask may not develop at its optimum speed. The developing equipment and process should be set up according to the solder mask manufacturer s recommendations. This includes the chemistry, pressures, flow rates, nozzle type(s), process cycle times, and so on. 33.4.4.2 Solder Dams ( Webs ). To reduce soldering defects in assembly, designers have placed fine solder dams, or webs, between attachment pads. The width of these dams is frequently in the 2.5 to 3.0 mil range, with some designs requiring dams as narrow as 1.0 to 1.5 mils wide. Producing features this fine is not possible with all solder mask materials, and the imaging process for very fine features will typically be modified from normal production with wider dams.
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Assembly Considerations 33.4.5.1 Thickness. When PCBs had all through hole components, solder mask thickness was not of much concern. However, with the advent of surface-mount technology in the 1980s, excessive solder mask thickness caused assembly problems, including:
Tombstoning of discrete devices in reflow soldering Excessive opens with discrete devices in wave soldering Solder paste printing problems (excessive paste, solder balls, paste on back of stencil, etc.)
Flip-chip assembly, with its finer pitch, has placed further restrictions on the maximum mask thickness. These thickness restrictions effectively eliminated dry film solder mask in most applications and relegated it to a specialty product for special applications. 33.4.5.2 Cleanliness. After PCBs have been assembled, they must meet specific ionic and visible cleanliness requirements. It is important that PCBs produced with the specific solder mask through the normal process be qualified through the assembly process to ascertain whether the production parts will meet expectations. Each solder mask and flux combination throughout the assembly process may give a different cleanliness result. 33.4.5.3 Solder Balls. Some assembly processes are more prone to generate solder balls than others. Prequalification of the solder mask with the assembly chemistries and processes is essential to predict production performance. If solder balls occur during assembly, in some cases they can be reduced by using a mask with a different surface finish. Satin or matte products sometimes have less of a tendency to have solder balls. Modifying the solder mask cure can have an effect on the level of solder balls also consult with the solder mask supplier before deviating from its recommended cure process. 33.4.5.4 Assembly Material and Process Compatibility. When qualifying a new solder mask or new PCB supplier, it is critical that all existing materials and processes be evaluated for compatibility before production is scaled up. This should help avoid major issues where there is a large quantity of PCBs awaiting assembly that cannot be assembled with the existing assembly materials and processes. Care should be taken to include evaluation of:
Soldering fluxes and pastes Cleaning agents Multiple heat exposures Adhesives Underfills Temporary solder masks Tapes Anything else in assembly that depends upon the surface of the solder mask
33.4.5.5 Conformal Coating Compatibility. Again, qualification of the solder mask, through all of the fabrication processes, is required to ensure that no problems with production boards arise. Since conformal coating is applied after assembly, a significant amount of processing and exposure to other chemistries will have happened since the solder mask has been applied. The conformal coating compatibility testing should be done on a board that has been through all of the assembly processing. The cleanliness of the PCB surface after the final assembly process has a great impact upon the adhesion of the conformal coating. One cannot
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